1. Field of the Invention
The present invention pertains to the manufacture of opthalmic lenses and in particular to bifocal opthalmic lenses.
Bifocal lenses require a distance correction, an astigmatic correction, an axis for the astigmatic correction ranging from 0.degree. to 180.degree., proper decentration, and a reading correction. Because of the nearly infinite number of possible combinations of these corrections, it has become both impossible and unfeasible for the laboratory to stock finished bifocal prescription lenses. Therefore, the manufacturer usually supplies the laboratory with a semi-finished blank. This blank has a finished front surface and a semi-finished rear surface, the rear surface being ground and polished to achieve the finished prescription, and the front surface containing a distance curvature and a bifocal addition. Conceivably, the manufacturer could, with the same effort, cast the finished prescription, but the economics of the industry do not permit the casting of individual prescriptions by the manufacturer, and, instead, he mass produces semi-finished blanks.
The laboratory, which is oriented to filling individual prescriptions, therefore, grinds and polishes the rear or concave surface of the concave-convex lens to conform to the finished prescription by taking into account the distance correction, the astigmatic correction, decentration, the axis of the astigmatic correction, and the bifocal addition. Ideally, it would be best for the laboratory to cast its own finished prescription rather than to buy the simi-finished blank, but the lack of technology at the laboratory operation and the economics of the operation simply do not permit the casting of individual prescription lenses.
Some of the problems involved in the casting of plastic lenses are that the plastic shrinks by about 15%, and that the mass of the cast plastic generates an exothermic reaction. The normal prescription requires casting two dissimilar curvatures, front and back, and this in turn may produce lenses having a greater center thickness and thinner edge thickness, or greater edge thickness and thinner center thickness. This further complicates the casting technique because the dissimilar center and edges must both shrink the same 15% and the technique used to have the molds conform to these dissimilar shrinkages is part of the art of casting plastic prescription lenses. Furthermore, the mass of plastic cast generates its own exothermic reaction and it is necessary during the initial portion of the curing cycle to introduce heat to intiate reaction and during the latter parts of the reaction to remove heat in order to keep the reaction under control. However, casting thin films and parallel surfaces can easily be accomplished since the exothermic reaction is negligible, the surfaces are substantially parallel, and the 15% shrinkage causes no appreciable reduction in mass. Thus, if a laboratory could make a finished bifocal lens by casting thin films, it might avoid the difficult task of grinding and polishing semi-finished blanks provided by the manufacturer.
2. Description of the Prior Art
Staehle (U.S. Pat. No. 2,339,433) discloses a method of adding a correction to a molded plastic lens. He does this by adding a thin level of resin. However, Staehle is only concerned with conventional-type lenses, those with only one focus, and furthermore, with changing the curvature of the lens. The bifocal lens, which has two foci, is not possible of production with Staehle's method. He does not disclose how two different foci can be effected by adding a resin layer to an existing lens.
Calkins (U.S. Pat. No. 3,946,982) discloses a method for casting multifocal lenses. However, this method requires two mold portions held together by a gasket, one of the molds having a recessed portion, so as to provide a bifocal effect. The liquid plastic is injected between the two mold portions, cured and cooled thus creating an entirely new lens, whereas applicant simply transforms an existing lens into a bifocal lens. Some of the problems with using gaskets are that they are expensive and that the assembly of the molds and gaskets requires expensive hand labor.
Naujokas (U.S. Pat. No. 3,248,460) discloses a method for casting multifocal lenses. However, this method utilizes the base blank whose curvature is only about half the curvature of the predetermined power of the composite lens. This requires, in order to complete casting the bifocal, the casting of a relatively large amount of mass of non-parallel surfaces which presents the ensuing problems of shrinkage and heat dissipation.